Obesity and weight gain are associated with increased morbidity and mortality, and affect a sizable and increasing population in the U.S. and other developed countries. Obesity is characterized by an increase in adipose tissue mass due to hypertrophy of existing fat cells and the generation of new adipocytes. It has been tacitly assumed that new adipocytes arise solely from resident adipose tissue preadipocytes or interstitial mesenchymal cells. However, preliminary data from my laboratory shows that bone marrow (BM)-derived circulating progenitor cells differentiate into a de novo population of adipocytes in the adipose tissue of mice. These novel adipocytes express conventional adipocyte markers, have multiple lipid droplets like brown adipocytes, but lack uncoupling protein-1, a brown adipocyte marker. These new multilocular (ML) adipocytes also exhibit high mitochondrial content, suggesting a potential role for these cells in oxidative fuel disposal. The number of ML adipocytes is increased by the thiazolidinedione rosiglitazone (ROSI) or high fat feeding. Based on these results we hypothesize that thiazolidinediones and high fat feeding increase the trafficking of mesenchymal progenitor/stem cells to adipose tissue and promote their differentiation to multilocular adipocytes that participate in adipose tissue function. Four Specific Aims will test this hypothesis. The first Aim will test whether BM-derived mesenchymal and hematopoietic progenitor cells give rise to ML adipocytes, and determine whether the progenitor cells exhibit stem cell characteristics. The second Aim will investigate the mechanism(s) by which TZDs and high fat feeding promote the appearance of ML adipocytes from BM- derived progenitor cells. Specific Aim 3 is a proof-of-principle aim that will determine whether BM-derived ML adipocytes from wild type mice can reconstitute leptin production in leptin-deficient recipient mice. Finally, Aim 4 will test whether ML adipocytes exhibit higher rates of fatty acid uptake, esterification and/or oxidation than conventional white and brown adipocytes. Successful testing of this hypothesis will demonstrate the importance of non-resident cells in the development of adipose tissue. We predict that the novel ML adipocytes will have a beneficial impact on adipose tissue function and global energy metabolism via the release of leptin and by increasing adipose fatty acid oxidation. These experiments should also reveal mechanisms for regulating the recruitment and differentiation of BM-derived progenitor cells to ML adipocytes. Such mechanisms could then form the basis for novel therapies to treat or prevent obesity, diabetes and related conditions. PUBLIC HEALTH RELEVANCE: Obesity and weight gain are associated with diabetes and cardiovascular disease, and affect a sizable and increasing population in the U.S. Obesity is characterized by an increase in fat tissue mass due to growth of existing fat cells and the generation of new fat cells. It has been assumed that new fat cells arise solely from progenitor cells that reside within fat tissue. However, preliminary data from my laboratory shows that bone marrow-derived progenitor cells can become new fat cells in the fat tissue of mice. The research proposed in this application will explore the mechanisms that control the formation of new fat cells from bone marrow- derived progenitor cells, and investigate the role of the new fat cells in the function of fat tissue. Successful testing of this hypothesis will demonstrate the importance of non-resident cells in the development of fat tissue. These experiments should also reveal mechanisms for regulating the formation of new fat cells from bone marrow progenitor cells. Such mechanisms could then form the basis for novel therapies to treat or prevent obesity, diabetes and related conditions.